Wavelength tunable lasers and widely tunable lasers are integral components of a range of instruments, devices, and tools use for characterizing or modifying materials, objects, or samples. Wavelength tunable lasers and widely tunable lasers are also used in sensors and for data transmission and communication. In many applications, the wavelength is tuned continuously (e.g. the wavelength is swept in time) and possibly according to a desired tuning profile. In other applications, the laser is tuned to a specific wavelength and parked or held predominately stationary at that wavelength for a length of time. There exists a need for highly flexible and configurable tunable or swept laser sources that can be adapted to the specific needs of a multitude of applications. Described here is a Laser Array that consists of a series of laser sources that are chosen to have specific properties so as to enable the Laser Array to be applied to a range of applications with improved performance or additional capability. Applications the Laser Array can be applied to include, but are not limited to: interventional medicine, basic research, biological research, laser based spectroscopy, medical imaging, biological imaging, industrial imaging, material characterization, morphological characterization of a sample, sample modification, sample stimulation, optical sensors, environmental sensors, health sensors, optical sensing, optical signal generation, data transmission, data communication, and interferometric measurement. The center wavelength for these Laser Arrays covers the entire optical spectrum from the deep UV to the Far IR, with the tuning range covering a few percent of the center wavelength out to tuning ranges of substantially more than a full octave. These examples are a subset of the multitude of applications where lasers, and more specifically tunable lasers have been found to be beneficial and it is understood that the current invention applies more generally beyond the specific applications stated.
Although certain embodiments of the present invention can be used in a wide range of applications, it is helpful to consider the embodiments in the context of a specific application. Optical coherence tomography (OCT) is an optical imaging technique that can be implemented using a wavelength tunable laser. Optical coherence tomography (OCT) is a non-invasive imaging technique that can provide high-resolution depth profiling or morphological characterization of a sample below or at the sample surface. OCT can also provide information about dynamic processes occurring in the sample by Doppler OCT, spectroscopic information about the sample by spectroscopic OCT, polarization and birefringent information about the sample through polarization sensitivity OCT, and precision measurements of deflection or motion through intensity and phase sensitive OCT. OCT will be used to provide an in-depth look at the benefits provided by the Laser Array described in this work. To add additional clarity examples will be presented based on the use of a VCL device, it should be understood that any tunable laser could be substituted for the VCL. In recent years, swept source optical coherence tomography (SSOCT) using wavelength swept lasers has demonstrated superior imaging quality, superior imaging range, and superior imaging speed relative to time domain and spectral domain OCT systems. The Microelectromechanical systems (MEMS)-tunable vertical cavity laser (MEMS-VCL) is expected to be a key swept source in emerging SSOCT systems, because of its truly single-mode mode-hop-free operation enabling long coherence lengths, and because of the short cavity and low MEMS mirror mass enabling MHz wavelength scanning rates. These advantages are described in (V. Jayaraman, J. Jiang, B. Potsaid, G. Cole, J Fujimoto, and A. Cable “Design and performance of broadly tunable, narrow linewidth, high repetition rate 1310 nm VCSELs for swept source optical coherence tomography,” SPIE volume 8276 paper 82760D, 20112.). Prior art OCT systems employing one VCL as a swept source have enabled MHZ rates and >100 nm wavelength sweep range. Nevertheless the imaging rates and sweep trajectories remain limited by the mechanical resonance and dynamic properties of a single MEMS actuator, and wavelength sweep range remains limited by the gain-bandwidth of a single semiconductor gain medium. Prior art OCT systems have also employed a MEMS-VCSEL with flat frequency response, enabling variable wavelength sweep rate in a single VCL and linearization of the sweep trajectory through adding harmonics of the fundamental drive frequency. In some applications, however, it is advantageous to operate at the MEMS mechanical resonance of the device, to take advantage of low voltage operation in a vacuum environment, as described (G. D. Cole, J. E. Bowers, K. L. Turner, and N. C. McDonald, “Dynamic Characterization of MEMs-Tunable Vertical-Cavity SOAs,” IEEE/LEOS International Conference on Optical MEMS and Their Applications (MOEMS 2005), Oulu, Finland, 1-4 Aug. 2005.)
From the foregoing, it is clear that there is significant benefit to a light source that could be used in a SS-OCT system employing MEMS-VCL or other laser technology in which sweep speed is not limited by MEMS mirror mechanical resonance, wavelength tuning range is not limited by gain bandwidth of a single semiconductor gain medium, and which exploits low voltage and possibly resonant operation.